Africa was a true terra incognito. Even as late as the 19th century, much of the interior remained unknown. It is strange how we could have forgotten our ancient roots. It had become the ‘dark continent’, a name that could only have been invented by someone who had never been there.
They missed out on wonders. Africa is a different world. We still feel at home in the savannah, our ancestral home. The animals and birds are out of this world. So is the geology: the great rift, the unique and ancient lakes, volcanoes of every sort and activity. There is Lake natron, and its volcano with carbon lava. The people are equal to the land, dynamic and complex. I have vivid memories of a stay in a village in the Malawi rift valley. It was an isolated village, without roads and with mud huts. There were only two stone houses. We stayed in one – the other was the church. You had to accept the local conditions. Hanging clothes out to dry meant supplying the village with them. Neither could you leave them on the inside table: the children were adapt at fishing through the windows. At night, it was best to stay inside because hippos were wandering through the village. Water was obtained from a crocodile-infested river. That was the reason I was there, visiting a friend who worked on a well-drilling project. There was a well there already, but it had been claimed by the herders with their animals, and the water was no longer fit to drink. The whole village came to help. In the evening the singing of the children gave an evocative atmosphere. Africa is truly different – it is alive.
The rift valley is one of the geological wonders of the world. It is a 3000 kilometer long scar which runs through the continent, from the triple point in Afar southward. Afar was the location of a flood basalt 30 million years ago. It was likely caused by a hot spot but whether this was a deep plume is disputed, as it is for many hot spots. The triple point joined three spreading centres. Two of these developed to the point where oceanic crust formed, splitting Arabia from Africa. This is common: where three spreading centres come together, usually only two will develop. The rift valley in Ethiopia has now dropped to below sea level, but that has taken 30 million years. As oceans go, it has been a bit of a non-starter. But what it lacked in vigour, it made up in length. It kept growing longer until it dissected Africa, running through Kenya and Tanzania, and on into Mozambique where it enters the ocean. It is considered as separating two plates: the African (or Nubian) plate and the Somali plate.
The main rift valley runs along the east side of Africa. But a western rift has also developed, running around Tanzania. In between the western rift and the eastern rift, a third plate has formed, called the Victoria plate. The western rift is also called the Albertine rift; it runs through Uganda, Rwanda and Burundi. Traditionally it was considered younger than the eastern rift. However, dating of deposits in the two valleys indicate that they have quite similar ages, 20 to 25 million years. The Victoria plate is the old craton of Tanzania, and it was too strong to break. The two rifts formed during a time of rapid uplift.
Africa’s volcanoes are associated with the rift valley. The famous Kenyan and Tanzanian volcanoes follow the eastern rift, although they are located outside the valley. Kilimanjaro and Ol Doinyo Lenga are found here. Mount Nyiragongo is in the western rift. The two rifts come together between Lake Tanganyika and Lake Nyassa (Malawi). Here is perhaps the least known, and most recent, of the world’s triple junctions. And here is the site of Africa’s most southerly active volcano: Rungwe.
Between Lake Tanganyaka and Lake Nyassa, the western rift consists of (from north to south) the Rukwa basin, the Songwe basin and the Karonga basin. The eastern rift forms the Usangu basin. There is some uncertainty whether the Usangu is a spreading centre or a transform fault. The Songwe basin is narrow, with on one side an escarpment of 1200 meters. The Usangu escarpment is slightly lower, at 1000 meters. The escarpment of the Karongo basin is called the Livingstone escarpment – I presume.
The oldest lava flows have been dated to 7 million years ago. The volcanic centers follow faults along the steep escarpment. The older flows and volcanic centers are deeply eroded. Three volcanoes are currently active: Ngozi, Rungwe and Kiejo. The Songwe and Usangu basins have 1-10 meter thick tephra layers which date from the holocene: these three volcanoes have been busy. The lack of vegetation at the summits also shows evidence of recent activity.
Of the three main volcanoes, Ngozi is the least impressive. It only rises some 300 meters above the valley floor and there is no obvious summit. However, it does have a caldera, partly filled with a lake. The lake is around 3km2 in area, and the caldera is a bit over 4 km2. The water in the lake has a high concentration of 3He, which suggests that the volcanic activity connects to the mantle. The crater formed in two eruptions 12,000 and 1,000 years ago. The latter is known as the Ngozi tuff. An eruption around 1450 left pyroclastic deposits 10 kilometer from the caldera. The main magma chamber is trachytic (intermediate between rhyolite and dacite), and 5-7 km deep. The chamber is capped by a layer of dense rock, formed by wetting the rock and baking. Ngozi can only erupt explosively! The composition of the ejecta suggets they are coming directly from the 5-km chamber, without any intermediate storage. This is similar to Hekla, and it makes it likely that eruptions develop fast without much wrning.
There are many hot water springs in the valley around Ngozi, with temperatures ranging from 30C to 90C.
Rungwe is the largest volcano, rising to 2956 m, or 1500 m above the valley floor. The summit is barren, highly notable in the surrounding forest. There is a 4 by 5 km caldera, open to the southwest. Pumice from Rungwe covers the entire local area. The most significant is the 4 ka Rungwe Pumice, traceable over more than 1500 km2. To the southeast, the pumice is found as far as 115 km distant. The isopach contours are nearly circular, suggesting that the plinian eruption took place during a period of little or no wind. The pumice deposit reached 3 meters of thickness near the summit, and is 50 centimeters 20 kilometers away. The total volume is 5 km3 at minimum. Several other eruptions have been dated to less than 1500 years.
Kiejo is different, It stands about 600 meter above the valley, is unforested, and it is build up by effusive eruptions. It is home to the only historically documented lava flow in the region, happening around 1840. The eruption lasted three days and the lava flows extended for 8 km.
Can we say more about the eruption rate? It turns out, quite a lot. The explosive events have left numerous ash deposits, and the lakes of the rift valley provided the ideal environments for their preservation. Obviously a close lake is better: Lake Tanganiyaka and Laka Malawi contain several ash layers but only for the largest eruptions. The best record is from Lake Masoko, some 25 kilometer away. It contains 70 tephra layers, going back 45,000 years. Some layers are less than 1 mm thick, others are much thicker, in one case reaching 1 meter (the Rungwe pumice of the 4 ka eruption). The average eruption rate is close to once every 500 years, and the typical size of an eruption is VEI4, occasionally 5. The youngest layer has been found in Lake Malawi: AD 1674 ± 13. The large 4 ka eruption came after a period of calm, and was followed by another period of quiescence. But since 1500 years, eruptions have been more frequent again.
Is this something to be concerned about? African volcanoes are not well monitored. There is no sign that an eruption is imminent, but is that because we aren’t looking? And the rift valley is densely populated. 300,000 people live within 20 kilometers of Rungwe. The houses could not withstand a significant ash deposit, and pyroclastics are also possible. Yes, it is worth monitoring the situation. This is best done remotely, using the sentinel missions, since equipment on the ground may not last long before it is used by enterprising locals.
The immediate area surrounding the volcanoes is a nature forest reserve. The Reserve is host to two of the world’s rarest primates, the Kipunji (Rungwecebus kipunji) and the Rungwe galago. But it also protects the human population against the vagaries of the volcanoes!
There is one more aspect which needs mentioning. Remember the open caldera of Rungwe? It turns out this isn’t caused by an explosion or a collapse. The signs are downslope. 16 km southwest of the summit is an area of low hills, around 7 meters tall, with a hummocky appearance, shown in the photo here. Such an area together with an open caldera is indicative of a flank collapse. It is called a debris avalanche deposit and it can be abbreviated ‘DAD’ if you so wish. The hummocks become taller towards Rungwe, reaching as high as 20 meter. The maximum runout distance is 22 kilometers, and the vertical fall is 1700 meter.
The age is not well determined. The DAD is overlain by the 4 ka Rungwe pumice and must be older. An upper limit comes from a radiocarbon date on the layer below. This sets the ages as between 4 and 25 ka.
The radiocarbon date is itself interesting, because it comes from a deposit that look very similar to DAD. It suggests there have been two such collapses, one around 25,000 years ago and one more recent. There is something unstable about Rungwe.
The DAD shows no indication of volcanic ejecta. The collapse did not involve an eruption. There may have been an earthquake or a rain storm, of course. If the latter seems speculative, the slopes of Rungwe receive 3 meters of rain yearly.
The Rungwe volcanic complex is the most southerly active volcano in Africa. As befitting its location in the terra incognita of the map above, it is little studied. But the study that has been done has shown that this is not a system to be ignored. There is danger here.
Albert, February 2019
This post was based on the paper The Rungwe Volcanic Province, Tanzania – A volcanological review. Karen Fontijn, David Williamson, Evelyne Mbede, Gerald G.J. Ernst, Journal of African Earth Sciences, 63, (2012), 12–31